Let drop_last modify `gather_for_metrics` (#2048)
* Drop last
* Test
* Uncomment out tests
* Update src/accelerate/test_utils/scripts/external_deps/test_metrics.py
Co-authored-by: Benjamin Bossan <BenjaminBossan@users.noreply.github.com>
* Document better
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Co-authored-by: Benjamin Bossan <BenjaminBossan@users.noreply.github.com>
